The following references disclose use of electrochemical cells:
U.S. Pat. No. 5,932,171
U.S. Pat. No. 4,481,086
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U.S. Pat. No. 4,493,760
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JP Application No. 10057297A
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EP Application No. 0983806A1
Electrochemical cells for use in water/wastewater treatment systems are designed to operate by making use of the water electrolysis process wherein, at the anode-water interface, OH—; being present in water due to electrolytic dissociation of water molecules donates an electron to the anode and can be thereby oxidized to oxygen gas which can be removed from the system. As a result, the H+ concentration can be enhanced at the anode-water interface so that H+ enriched acidic water can be produced. In a similar manner, at the cathode-water interface, H+ accepts an electron from the cathode and can be reduced to hydrogen to form hydrogen gas which can be similarly eliminated from the system so that the OH—; concentration can be increased at the cathode-water interface whereby OH—; enriched alkaline water can be generated. Further, when a halogen-containing water (such as, natural water containing sodium chloride or an aqueous solution of sodium chloride) is subjected to electrolysis, halogenated mixed oxidants are generated in the electrolyzed water. This process can be further enhanced by adding selective precursor chemical substances in liquid, gas or solid phase upstream of the electrolytic cell so that upon entering the electrolytic cell they are dissociated or re-combined into tailored amounts of specific oxidants, reductants, or reactants. Ultrasonic treatment when used in conjunction with electrochemical cells can enhance the production of hydroxyl radicals and other oxidants, can keep the cell electrodes free of carbonate, sulfate, sulfide and iron oxide deposits, create a more thorough mixing in the electrochemical volume being treated, reduce the ion boundary layer thickness on each electrode, and either create micro- or nano-bubbles for increased surface area and longer life, or to agglomerate bubbles to enhance flow within the cell. On large water treatment processes electrochemical cells are usually used to treat a side stream of the main flow which is then recombined with the main flow to effect the desired water treatment, or on reduced flow systems the full flow can be directed through the electrochemical cell(s) for treatment.
Water quality can be defined by measuring the concentrations of oxidants, total hardness, total dissolved solids, total dissolved organics, chemical oxygen demand (COD) biological oxygen demand (BOD), specific contaminants such as heavy metals, pharmaceuticals and/or pathogens, industrial compounds, hormones and other endocrine compounds, dissolved oxygen, conductivity, oxidation-reduction potential (ORP), streaming current potential, and turbidity of the water.
Drinking water supplies are commonly disinfected with an oxidizer like chlorine. However the organics in the raw water mix with the chlorine used for disinfection to create cancer-causing agents like trihalomethanes (THMs) and haloacetic acids (HAAs). Drinking water and wastewater treatment plants may use on-site electrolytic generators to produce the chlorine used for disinfection and/or ultraviolet light for disinfection and/or as part of an advanced oxidation system for targeted organics destruction. By minimizing the organic content of the water it is possible to reduce the THM/HAA production and create a better quality water supply. Swimming pools, spas, water features such as ornamental fountains and the like are commonly sanitized using either electrolytic chlorination with/without an ultraviolet light clarifier, or ozonation. Each of these technologies has its own distinct advantages and disadvantages.
Conventional apparatus used to sanitize water in pools and the like, includes electrolytic chlorination systems, or “salt” chlorination systems. These systems utilize an electrolytic cell or “Chlor-alkali” cell, typically comprising a submerged positively charged anode, a negatively charged cathode, and an electrical energy source for applying a current across the gap between the anode and cathode. The anode compartment contains an anolyte including a source of chlorides which, when oxidized, forms chlorine gas. Typically, the chloride source comprises an alkali metal chloride salt such as sodium chloride or potassium chloride, although other sources, such as hydrochloric acid and the like may also be used.
When current is applied across the anode and cathode gap, the sodium and chloride ions disassociate with chloride ion concentrating in the anolyte solution and the sodium ion concentrating in the catholyte solution. Chlorine and/or oxygen gas is generated on the anode surface and hydrogen gas is generated on the cathode surface which is released back into the flowing water. The dissolved chlorine gas reacts with the water to create hydrochloric acid (HCl) and hypochlorous acid (HOCl). When either ozone or hydrogen peroxide are added as precursor compounds the electrochemical cell, or electrolytic device will produce small amounts of chlorine dioxide in addition to chlorine and other mixed oxidants. At concentrations greater than 1 ppm, hypochlorous acid minimizes or prevents the growth of algae, bacteria, and other microorganisms. When an electrolytic cell is used, the sodium hydroxide and hypochlorous acid recombine to form sodium hypochlorite (bleach) which is the active oxidizer transported back into the main body of water to prevent microorganism growth. Typical examples of salt chlorination systems are disclosed in Kosarek, U.S. Pat. No. 4,361,471, Wreath, et al., U.S. Pat. No. 4,613,415, and Lynn, et al., U.S. Pat. No. 5,362,368, the entire disclosures of which are incorporated herein by this reference.
One shortcoming of the electrolytic cell is that calcium carbonate or sulfate scale and bio-film build up on the cathode side of the mono- or bi-polar cells with time. The carbonate ion is created from the oxidation of organic matter with the chlorine sanitizer and it combines with the calcium ion in the water to make calcium carbonate salt. Elemental iron in the water is oxidized to iron oxide which coats the electrode surface and provides sites for the hardness scale to attach itself to the electrode. Current electrolytic cell technology reverses the polarity to switch the anode and cathode surfaces on the bipolar plate to dissolve the calcium carbonate scale build up on the alternate side of the plate. Large scale pieces and organic material build up in the electrode pack and usually need to be removed with acid cleaning or via the addition of surfactants.
Another shortcoming of the electrolytic cell is that it produces a constant chemistry in the cell and so if the water quality changes in the body of water being treated, such as increased bather load, or a slug of organic material enters the water volume (and the ORP changes), then the sanitizer chemistry may be overcome leading to an unsafe condition for human health either temporarily, or for an extended period of time until the sanitizer chemistry catches up with the demand.
Another shortcoming of the electrolytic cell is that it produces a constant chemistry in the cell that is independent of the type of water being treated and the particular contaminants in that water. For example, the water treatment chemistry required via electrolytic generation for a pool, is very different from the treatment required for remediation of contaminated groundwaters. In each of these cases, additional compounds are added manually to effect the desired treatment. Eg. in a pool environment, superchlorination with monopersulphate is required to destroy chloramines.
Another maintenance problem with electrolytic chlorination systems is that they are not particularly effective on algae reduction and so the addition of algaecides and the like must be included in the maintenance routine for pool operators. This is usually a temporary condition and the algae problem goes away upon treatment.
Another shortcoming of electrolytic chlorination systems is that amines, such as ammonia, tend to build up in the water over time, binding with the chloride to form chloramines. Since chloramines have strong odors, can irritate the skin and eyes of bathers, are toxic to ingest, cause discoloration and fading of human hair and bathing suits, it is recommended that pool and spa owners periodically superchlorinate or “shock” the water by adding high amounts of chlorine. The increased chlorine breaks down the chloramines by oxidizing the amines to nitrogen gas. Unfortunately, the amount of chlorine required for superchlorination is higher than the safe concentration for swimming or bathing, thus rendering the pool unusable for an extended period.
Another recommended option for removing chloramines, bacteria, viruses and protozoa from commercial pools and the like is to install an ultraviolet (UV) lamp disinfection system upstream of the electrolytic chlorination system. The UV disinfection system uses low-pressure, high-output mercury lamps or medium-pressure mercury lamps contained in individual quartz sleeve to treat the saltwater flowing through the cell. The UV radiation from the lamp(s) decomposes the chloramines into hydrochloric acid and nitrogen gas. The UV radiation inactivates the microbial DNA of bacteria and algae which makes the microbes more susceptible to chlorination. The UV disinfection system is a relatively high maintenance item, because the quartz sleeve(s) have to be cleaned regularly to prevent particulate build up on the sleeve which would block the UV radiation. Currently, a mechanical-chemical wiper system is used to remove soft scale from the quartz sleeve.
Conventional apparatus for sanitizing water using ozonation typically comprises a high efficiency ozone generator and a venturi mixer or inductor port that injects ozone gas into the water to oxidize contaminants in the water. Exemplary ozonation systems which have been found to be particularly effective in pools and spas are disclosed in Martin et al, U.S. Pat. No. 6,500,332, Martin et al, U.S. Pat. No. 6,129,850, Martin et al U.S. Pat. No. 6,372,148, and Martin, U.S. Pat. No. 6,331,279. Other ozonation systems are disclosed in Karlson, U.S. Pat. No. 5,855,856, Morehead U.S. Pat. No. 5,451,318, Engelhard, U.S. Pat. No. 5,709,799, and Karlson et al., U.S. Pat. No. 5,518,698. The entire disclosure of each of these patents is incorporated herein by this reference.
Ozone has been recognized by the FDA to be more than 200 times stronger than chlorine in microbial kill, and can react at higher oxidation levels than can be achieved safely with chlorine. However, dissolved ozone can exist in water for only a very short period before it reacts and is converted back into oxygen gas. Thus, dissolved ozone is not an effective residual sanitizer, in contrast to chlorine which has relatively steady and consistent residual sanitation properties.
To overcome the short residence time of ozone and the high vapor pressure of chlorine in hot spa water, spa and pool owners have added at sodium bromide salt to the water. Bromine has a very low vapor pressure compared to chlorine, thus, it does not vaporize as readily in aerated hot spa water. Dissolved ozone or sodium hypochlorite will react with the bromide ion to create the hypobromite ion in the water. Hypobromous acid or sodium hypobromite salt will oxidize ammonia to nitrogen gas without creating an intermediate amine compound like the chlorine oxidizer.
Attempts to combine the favorable properties of chlorination and ozonation are described in Tamir, U.S. Pat. No. 4,804,478 and Gargas, U.S. Pat. Nos. 6,517,713, 6,551,518 and 6,814,877 B2. The entire disclosure of each of these patents are incorporated herein by this reference.
In the evolution of water treatment it has been identified that organics in the water are undesirable and lead to the formation of carcinogenic compounds when chemically reacted with sanitizers, and disinfecting agents like chlorine, bromine and ozone. For this reason it is desirable to reduce as much as possible the concentrations of organic compounds in the water. For this reason, advanced oxidation processes have been developed to destroy the organic compounds before they can react with the sanitizing/disinfecting agents. Advanced oxidation processes (AOPs) are defined as those processes that optimize the production of hydroxyl radicals (OH) and oxygen species without the addition of metal catalysts. In water treatment, AOPs refer specifically to processes where oxidation of organics by hydroxyl radicals (OH—) occurs specifically through processes that involve ozone (O3), hydrogen peroxide (H2O2) and/or ultraviolet light (UV with λ<300 nm), Fenton oxidation, and sonolysis. All AOP systems generate OH radicals via a pressure (cavitation), chemical reaction, electric field, or photon-based process, or combinations thereof. The ability of an oxidant to initiate chemical reactions is measured in terms of its oxidation potential. The end product of complete oxidation (mineralization) of organic compounds is carbon dioxide (CO2) and water (H2O). The oxidation potential of OH radicals at 2.8V is high relative to ozone at 2.1V and chlorine at 1.4V.
Depending on the existing oxidants in the water and whether salts, anions, ozone and/or air are added to the water a number of other oxidizers may be generated under AOP conditions including: ozone, peroxomonosulfuric acid, peroxodisulfuric acid, sodium peroxycarbonate, peroxodiphosphate and hydrogen peroxide, all good disinfectants and oxidizers. In general these peroxides can also kill micro-organisms, however these peroxides are very unstable. Perborates are very toxic and peracetic acid (PAA) is a strong acid. PAA can be aggressive in its pure form. Stabilized persulphates can be used to replace chlorine to meet “chlorine-free” disinfection requirements as can electrolyzed water processes.
AOP systems are designed to treat a wide range of common water pollution problems, for example: total organic carbon (TOC) removal in high purity water systems such as pharmaceutical and semiconductor manufacturing, N-Nitrosodimethylamine (NDMA), a contaminant found in groundwater from liquid rocket fuel production or as by-product of rubber processing, other groundwater pollutants such as Methyl-tert-butyl ether (MTBE), trichlorethylene (TCE), acetone, phenols, benzene, toluene, and xylene. Pesticide removal such as atrazine and 1,4-dioxane from surface or groundwater supplies and bromate removal caused by ozonation of water containing bromide ion are other AOP processes. Dechlorination and dechloramination of process water is another AOP process.
Conventional AOP technologies are fairly well understood and straightforward to design and implement. Some of the newer AOP technologies such as: TiO2 catalyzed UV oxidation, electro-hydraulic cavitation, electrochemical oxidation, UV/electrochemical oxidation and streaming current electric discharge (SCED) have the potential to deliver greater efficiencies and better performance than conventional treatment processes with the caveat that each contaminant cocktail is different and must be evaluated for the most appropriate treatment alternative.
In the AOP system the chemical reactions are highly accelerated oxidation reactions that occur when the OH radicals react with organic pollutants to initiate a series of oxidative degradation reactions. However, OH radical inevitably reacts with all kinds of organic and inorganic constituents in water which result in decreasing the efficiency of OH radical for degrading the pollutant of interest. Dissolved iron oxidation uses the OH radical before the oxidation of organics. It is also known that high alkalinity reduces the OH concentration preferentially to generate the carbonate ion. Therefore, the biggest issue of AOP process lies in increasing the OH production yield and directing the reaction pathway where major reactions between OH radical and the pollutants occur.
There still exists a need for an electrolytic water treatment system that can operate as a combined advanced oxidation process—residual oxidant generator for treatment of a wide range of water qualities and uses. There exists the need to be able to generate a unique oxidant mix (or reductant, or reactant mix) via the addition of precursor compounds and materials upstream of the electrolytic cell such that the resulting electrolytic chemistry contains sufficient numbers of particular and varied oxidizing/reducing species as is necessary to effectively treat the water contaminants. Furthermore there is a need for the addition of precursor chemicals and materials to optimize the electrochemical output of the cells with products that are more useful to the particular water treatment application. Furthermore, there exists a need for such systems which can be manufactured simply and inexpensively, which can easily fit or be retrofitted into a conventional drinking water plant, swimming pool, spa, cooling tower, water feature or the like, and which requires relatively little maintenance.
There still exists a need for an electrolytic water treatment system that can operate as a combined advanced oxidation process—residual oxidant generator for treatment of a wide range of water qualities that uses an ultraviolet lamp(s) as a virtual anode(s) in an electrolytic cell and a separate wire(s), or a surface of the cell, as the cathode(s). There exists the need to be able to generate a unique oxidant mix, or reductant mix, or reactant mix via the addition of precursor compounds or materials upstream of the UV— electrolytic cell such that the resulting electrolytic chemistry contains sufficient numbers of particular and varied oxidizing/reducing species as is necessary to effectively treat the water contaminants in real time and under changing, generally degrading, water quality conditions. Furthermore, there exists a need for such systems which can be manufactured simply and inexpensively, which can easily fit or be retrofitted into a conventional drinking water plant, industrial treatment plant, swimming pool, spa, cooling tower, irrigation channel, mining process, water feature or the like, and which requires relatively little maintenance.
There still exists a need for an on-site electrolytic/electrochemical based mixed oxidant and/or sodium hypochlorite generator using only salt, water, electricity and custom prepackaged, or bulk precursor compounds to create custom liquid admixtures for municipal water treatment, industrial water treatment, oil & gas produced water remediation, solution mining, mine wastewater cleanup, cyanide destruction, acid mine drainage cleanup, or like applications. The mining industry currently uses bulk solutions of sodium hypochlorite for solution mining having a single pH value. The flexibility and enhanced process performance afforded the solution mining operation and/or mine wastewater remediation operation with the present invention is significant due to the varied pH and electrolytic chemistries that can be produced at will.